The present invention relates to IFF systems, and more particularly to techniques for determining the occurrence of a target's "leading" and "trailing" edges.
The position with respect to an IFF antenna where the IFF system first reliably receives acceptable replies from a target is generally termed the target's "leading edge." Similarily, the "trailing edge" is generally considered to be the target's angular position where satisfactory replies are no longer reliably received.
In general, most existing IFF (secondary radar) equipment determines "leading" and "trailing edges" through either sliding window or run determining techniques. Basically in a sliding window type technique received replies are examined to determine if a number T.sub.L of correct replies exist in a number W.sub.L of replies to interrogations. This technique also examines replies to declare a "trailing edge" when a number T.sub.t of incorrect replies are received in a group of W.sub.t replies to a number U.sub.t of interrogations. In contrast, run determining techniques examine the received replies for a run, or sequence, of a predetermined number R.sub.L of correct replies received, and declare a "trailing edge" only when interrogations elicit a run of R.sub.t replies which are not correct replies. A "leading edge" is determined in the converse manner. Still other existing techniques makes use of up-down counters in order to determine when a "leading edge" or "trailing edge" exists.
Once a leading and trailing edge has been declared, it is important to determine the target azimuth. In prior IFF equipments, the azimuth of a responding target has generally been estimated by finding the difference between the antenna where a "trailing edge" was declared and where a "leading edge" was declared, and then estimating the target's position by adding half the difference between the two azimuths to the "leading edge" azimuth.
This method of determining target azimuth has two basic faults: first, since a "trailing edge" is usually declared very quickly--anytime a few interrogations fail to elicit correct replies--various types of interference may cause a "trailing edge" to be prematurely and spuriously declared. An early spurious "trailing edge" declaration is usually accompanied by a second and erroneous "leading edge" declaration, and the consequent display of the one target as two. Second, all target azimuths estimated in this manner are biased slightly in the direction of the interrogator's antenna rotation. This bias, being essentially the same for all targets (and generally less than a degree in current systems) has not yet constituted a significant operational problem. In further systems however, where interrogation repetition rates may be reduced and more precise aircraft control required this angular bias error may become an important factor to be reckoned with.